As the nervous system develops, synaptic connections between nerve cells are initially established in a highly specific manner. These initial contacts can then be modified to a limited extent during early postnatal life, increasing their specificity still further. A knowledge of the functional and morphological correlates of synaptic development in the central nervous system is basic to an understanding of how nerve cells originally make and then refine and maintain their proper connections. Synaptic connections in the spinal cord between muscle sensory afferent fibers and motoneurons, connections which form the neuronal basis of the stretch reflex, provide an excellent experimental system for studying this problem at the level of single, functionally identified cells. In the frog's spinal cord, these connections form with a high degree of specificity. Moreover, sensory fibers can regenerate after they are cut in the dorsal root, and they appear to make synaptic contacts in the appropriate regions of the spinal cord. A study of the factors that influence the specificity of this regeneration could help in achieving specific regeneration in the human spinal cord after accidental injury. My objectives during the next five years are as follows: 1) Study, using anatomical and electrophysiological techniques, the monosynaptic input that motoneurons receive from single, functionally identified sensory axons in normal adult animals. 2) Extend this analysis of connections made by single cells to the formation of sensory-motor synapses during development. 3) Examine the anatomical and physiological specificity of reflex connections made by regenerating sensory axons after interruption of a dorsal root.